This invention relates to proportional control devices, and more particularly, to proportional actuators for proportional control devices.
Proportional control devices include a proportional actuator for positioning an armature within the case of the proportional control device. The proportional actuator causes the output of the proportional control device to be related directly to the applied current and independent of a function being controlled. For example, one application of proportional control devices is in control of fluid flow. In such application, a proportional control valve monitors pressure and the rate of flow of fluid through the valve is proportional to the magnitude of the applied current and is independent of changes in pressure of the fluid.
Typically, proportional actuators include a solenoid coil which is wound on a stationary magnetic pole. Current applied to the solenoid coil creates an attractive field in the stationary pole for moving an armature to operate a spool valve for communicating a supply port with a control port. The response time of the proportional actuator as well as the turn-on threshold are a function of the amount of force produced by the device. The amount of force which can be generated by proportional actuators of this type is related to the coaxial diameters of the magnetic pole and the armature, the number of turns of solenoid coil and the current that is applied to the solenoid coil. The solenoid coil size generally determines the dimensions of the device because the solenoid coil is wound on the magnetic pole. Thus, methods of maximizing the force generated by such devices are usually directed to optimizing the magnetic circuit of the device.
For example, the operating efficiencies of proportional actuators can be increased to some extent by improving the magnetic flux coupling between the magnetic pole piece and the armature. To this end, an element, which is commonly referred to as a saturation tip, is provided for directing magnetic flux through the pole piece to the armature. The saturation tip bridges the gap that exists between the opposing surfaces of the armature and the pole piece when the device is not actuated. In known proportional actuators, the saturation tip is formed on the stationary pole piece and the armature moves axially within the saturation tip. This construction, together with the configuration of the solenoid coil of such actuators, limits the coaxial diameters of the pole piece and the armature, thereby limiting the amount of force that can be generated for an proportional actuator of a given size.
In some known proportional actuators, the saturation tip was provided by brazing a tubular member of a magnetic material to the stationary magnetic pole piece as an extension of magnetic shoulder portion of the pole piece. In another known proportional actuator, which is disclosed in U.S. Pat. No. 5,377,720, the magnetic flux is directed by brazing the pole piece to a non-magnetic stainless steel sleeve which, in turn, is brazed to the valve body. However, the brazing operations required by these prior art arrangements add to the cost of the proportional actuator.
Proportional actuators can be incorporated into various types of control devices, and are particularly suitable for application in proportional fluid flow control devices for providing a proportional relationship between applied current and the fluid flow output of such devices. One problem associated with known proportional control valves is their tendency to overshoot a target position. Typically, such valves incorporate some type of damping mechanism which minimize overshoot, but slow the response time of the valve.